In a conventional electric discharge machining, relative motions are imparted to an electrode and a workpiece in a direction to force the former into the latter thereby to effect the machining operation while ensuring such a servo control that the gap between the aforementioned electrode and workpiece is usually maintained constant in that direction. In the usual electric discharge machining, here, after the rough machining process, a plurality of electrodes having similar shapes but slightly different sizes are used to perform the finishing process. This is partly because the rough machining process has a high machining rate but provides a rough surface whereas the finished process provides a fine surface but has a low machining rate and partly because the side gap between the electrodes and the workpiece is narrower for the finishing process than the rough machining process.
With a view to effect the rough machining process and the finishing process by means of a single electrode, therefore, there has been proposed an apparatus, by which motions having a component perpendicular to the usual feeding direction, e.g., revolving motions are imparted to the electrode and the workpiece, after the rough machining process, so that the finishing process may be accomplished by means of the electrode used for the rough machining process in a similar manner as the case in which an electrode having an apparently larger size is used.
The apparatus thus proposed is shown in FIG. 1, for example. A workpiece (2) is machined by arranging an electrode (1) and the workpiece (2) in an insulating liquid in a manner to face each other and by supplying the machining gap in-between with a pulse current from a pulse current supplier (3). At this time, the electrode (1) is fed in the direction (of an axis Z) toward the workpiece (2), e.g., in a manner for a voltage V.sub.d at the machining gap to averagely coincide with a standard voltage V.sub.s by means of both a servo-circuit composed of a voltage differential circuit (4) and an amplifier (5) and a servo-mechanism composed of a hydraulic servo-valve (6) driven in response to the output signal of said servo-circuit and a hydraulic cylinder (7), thus advancing the machining operation. After the rough machining operation has been ended up to such a depth as is set at a slightly shallower position than the desired target depth, the energy of the single pulse of the pulse current supplier (3) is changed to have a smaller level, and X-Y cross tables (11) and (12) are connected by moving servo-motors (9) and (10) in a known manner by an electrode motion controller (8) thereby to perform a revolving motion. In this instance, it is sufficient the servo-motors (9) and (10) be supplied with sine waves having such voltages as have their phases made different .pi./2 and have their amplitudes corresponding to the difference in the side gap between the aforementioned rough machining process and finishing process. Then, the machining process is performed again up to the desired depth while effecting the relative revolving motions of the electrode (1) and the workpiece (2) in the aforementioned manner. Since, in this instance, there can be obtained an effect equal to the case in which the electrode (1) is enlarged to the size corresponding to the diameter of the relative revolving motions, the roughnesses left as the result of the preceding rough machining process can be removed.
Now, in case, with the use of the electrode (1) having such an oval cross-section as is shown in FIG. 2, the workpiece (2) is machined with the corresponding hole by the aforementioned method, the extent to which the workpiece (2) is machined for each portion of the locus of the revolving motions is remarkably larger at the portion where the electrode (1) has larger radius of curvature than at the portion where the electrode (1) has a smaller radius of curvature. Consequently, as the machining operation proceeds, the depth of the machining operation at the portion having the larger radius of curvature becomes larger than that at the portion having the smaller radius of curvature so that the depthes of the machining operations become considerably different.
Therefore, the conventional apparatus thus constructed has notable drawbacks that, for a deep machining operation, the roughnesses resulting from the preceding rough machining process cannot be sufficiently cut off and that the depth which can be reached by the electrode during the finishing process becomes different for the respective portions in accordance with the shape of the electrode.